Magnetic recording media are widely used as recording tapes, video tapes or floppy disks. A magnetic recording medium is fundamentally comprised of a non-magnetic support having laminated thereon a magnetic layer containing ferromagnetic particles dispersed in a binder.
High levels of various characteristics such as electromagnetic characteristics, running durability and running efficiencies are necessary for a magnetic recording medium. Specifically, an audio tape must have a high capability for reproducing original sounds in order to produce a high quality recordation and reproduction of recorded music. In addition, a video tape must have excellent electromagnetic characteristics in order to produce an excellent reproduction of original images.
As described above, it is necessary for a magnetic recording medium to have not only excellent electromagnetic characteristics, but also excellent running durability. In this connection, an abrasive agent generally plays an important role in order to obtain good running durability. Such an abrasive agent is distributed wholly in a magnetic layer, and a part of the abrasive agent is present on the surface of the magnetic layer. When a magnetic recording medium runs, it makes contact with various running parts such as a tape head. The abrasive agent present on the surface of the magnetic layer exhibits abrasive effects at the contact surface between the surface of the magnetic layer and the head. Accordingly, it is possible to improve running durability by incorporating an abrasive agent in a magnetic recording medium. However, when an abrasive agent is added in the magnetic layer, only a small portion of that abrasive agent is present on the surface of the magnetic layer, and it is difficult to obtain sufficiently excellent running durability with such a small portion of the abrasive agent on the surface. Therefore, it would appear at first glance that more abrasive agent should be added to increase its surface presence in order to improve running durability. But, when an increased amount of the abrasive agent is added to improve running durability, the content of ferromagnetic particles decreases and electromagnetic characteristics deteriorate. Moreover, when abrasive agents having a large particle diameter are used, the abrasive agents are likely to excessively extrude on the surface of the magnetic layer. Further, experiments have been conducted using abrasive agents having a large particle diameter and those having a small particle diameter in combination, but sufficiently excellent electromagnetic characteristics have not yet been obtained. See JP-A-57-162129 and JP-A-58-85931. (The term "JP-A" as used herein means an "unexamined published Japanese patent application").
In order to solve the above problem, it has been proposed that a magnetic layer be divided into two magnetic layers: A lower layer and an upper layer. According to this proposal, the upper layer would exhibit excellent running durability and the lower layer would have excellent electromagnetic characteristics, whereby running durability would be improved without deteriorating electromagnetic characteristics. See JP-A-58-200425. That is, it was designed to improve electromagnetic characteristics by making the content of ferromagnetic particles large without adding additives such as abrasive agents into the lower layer, and it was designed to improve running durability by (i) making the thickness of the upper layer from 0.5 to 1.5 .mu.m, and (ii) using an abrasive agent which has maximum particle diameter not exceeding the thickness of the upper layer.
By providing two magnetic layers as described above, a magnetic recording medium's electromagnetic characteristics as well as its running durability can actually be improved to some extent. But, despite this actual improvement, running durability is still not sufficiently satisfactory.